Fighters with the militant group the Islamic State in Iraq and Syria reached the triangle border between Iraq , Syria and Turkey , it said in a message posted on Twitter on Sunday .

ISIS took control of Iraq 's largest hydroelectric dam on Iraq 's Tigris River , which provides power to the city of Mosul about 50 kilometers -LRB- 31 miles -RRB- to the south , the commander of the Peshmerga Kurdish fighters who had been defending the facility said Sunday .

The dam workers remained inside the facility , which fell after a 24-hour battle , Lt. Col. Herash said .

A `` horrendous prospect ''

ISIS -- known for killing dozens of people at a time , while carrying out public executions , crucifixions and other acts -- has taken over several cities as it seeks to create an Islamic caliphate that encompasses parts of Iraq and Syria .

Daniel Pipes , the president of the Middle East Forum , said seizing dams is a tactic the group uses to gain control of a town and its people .

No dam , however , is as formidable or as important to Iraq as the one in Mosul .

`` If you control the Mosul Dam , you can threaten just about everybody , '' Pipes told CNN 's Jonathan Mann .

Pipes said the militant group now has the potential to create a flood so massive and catastrophic that it would not only cause death , destruction and chaos locally in the city of Mosul , but more than 450 kilometers -LRB- 280 miles -RRB- away in Baghdad as well .

`` It 's a horrendous prospect , '' he said .

Peshmerga Kurdish fighters also pulled out of the towns of Zumar and Wana after being surrounded by ISIS fighters and isolated from any support , Kurdistan Democratic Party regional official Ismat Rajab told CNN on Sunday .

FAA restricts flights over Iraq as conflict with ISIS rages

The United Nations in Iraq warned that 200,000 civilians were trapped in a dire circumstances after ISIS and associated armed groups `` seized control of nearly all of Sinjar and Tal Afar districts in Ninewa Province , including the oil fields of Ain Zala and Batma , bordering the Kurdistan Region of Iraq . ''

Most of the refugees are from the Yezidi sect and have fled to Jabal Sinjar , the United Nations said .

`` The humanitarian situation of these civilians is reported as dire , and they are in urgent need of basic items including food , water and medicine . An unknown number of civilians are also reported to have moved towards Dahuk and Zako in the Kurdistan Region . ''

ISIS to Christians in Mosul : Convert , pay or die

ISIS took control of Sinjar , a small town inhabited by the Yezidi sect , on Saturday , according to police officials .

An ISIS Twitter posting linked to a statement referencing `` the battle of opening the borderline between state of Ninawa and Dohuk provinces . ''

`` The Islamic Caliphate legions have launched since this morning '' operations towards the northwestern regions bordering state of Nineveh , '' the statement said . `` God facilitated for the mujahedeen to break into many important areas controlled by the Kurdish gangs and secular militias . ''

The State Department said Sunday that it was `` actively monitoring the situation '' in Sinjar and Tal Afar , and said that the U.S. is supporting both Iraqi security forces and Peshmerga forces in the fight against ISIS . `` The -LRB- ISIS -RRB- assault over the past 48 hours on territories along the border of the Iraqi Kurdistan Region and focusing on towns and villages populated by vulnerable minorities , demonstrates once again that this terrorist organization is a dire threat to all Iraqis , the entire region , and the international community , '' spokeswoman Jen Psaki said in a statement .

U.S. agrees to send 5,000 more Hellfire missiles to Iraq

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U.N. warns 200,000 civilians are trapped in a dire circumstances after ISIS gains

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`` They are in urgent need of basic items including food , water and medicine , '' the U.N. says

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ISIS controls the country 's most important dam on Iraq 's Tigris River

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Analyst : ISIS controlling the Mosul Dam is a `` horrendous prospect ''